Method of coating webs with photographic emulsions or other liquid compositions utilizing an electric field



Aug. 19, 1969 w, DE 5555' ET AL 3,462,286

METHOD OF COATLNG WEBS WITH PHOTOGRAPHTC EMULSIONS OR OTHER LIQUID COMPOSITIONS UTILIZING AN ELECTRIC FIELD Filed July 16, 1963 United States Patent 3,462,286 METHOD OF COATING WEBS WITH PHOTO- GRAPHIC EMULSIONS OR OTHER LIQUID CiOMPOSITIONS UTILIZING AN ELECTRIC F ELI) Wilfried Florent de Geest, Berchem-Antwerp, and Paul August Verkinderen, Edegem, Belgium, assignors to Gevaert Photo-Producten N.V., Nortsel-Antwerp, Belgium, a company of Belgium Filed July 16, 1963, Ser. No. 295,549 Int. Cl. G03c 11/00; (103g; B05b 5/02 US. Cl. 117-34 This invention relates to a method of coating liquid compositions and, more particularly, to an improved method of coating liquid photographic emulsions onto supports.

In the process of coating webs of materials such as photographic papers or film supports with liquid photographic emulsions, difiiculty is experienced if one attempts to employ a high coating speed of the web.

One difficulty which frequently occurs is the repellency of the coating by the support.

Another difficulty which frequently occurs is the partial rupture of the coating bead. 'By coating head is understood, the continuous liquid mass which is present between the supply of the coating composition, and the support.

Many suggestions have been offered for eliminating the said difficulties, and the addition of surfactants and other types of spreading agents to the composition being coated, are the best known ones.

While the addition of the said agents leads to satisfactory results in certain instances, their use puts serious problems in the manufacture of photographic products because of the photographic effects of such added agents.

Another serious problem encountered in coating photographic emulsions on support materials is the presence of static charges on the support materials. The said charges are irregularly distributed in the support and all over its surfaces, and thereby may lead to disturbances in the coating head.

In US. patent specification No. 2,952,559, Gale F. Nadeau discloses an improved method of coating photographic emulsions which will overcome the above-mentioned difficulties on the average. His method comprises the steps of neutralizing the polarization charges in the support by passing the support through the bite of a pair of opposed grounded metal rollers, rendering the surface of said support electrically neutral by directing a flow of ionized air onto said surface, applying a uniform electrostatic charge to said surface to facilitate the even spreading of the liquid emulsion on said surface, applying an electrostatic charge of opposite polarity to a supply of said liquid emulsion, and transferring said liquid emulsion from said supply to said surface.

While the above-mentioned method results in higher coating speeds and less disturbing effects in coating, it has disadvantages which the present invention is intended to overcome.

One disadvantage resides in the high tension which in practice has to be applied to the hopper and which puts serious constructive insulation problems. Further, extensive measures have to be taken for duly screening the high tension supporting parts.

Another drawback is the use of a flow of ionized air which is directed to the support prior to coating. In case a second photographic emulsion or e.g. an anti-stress layer has to be coated after a first photograph layer has been applied, the use of said ionized air may give rise to fogging of the first coated photographic layer.

8 Claims 3,462,286 Patented Aug. 19, 1969 One object of the present invention is to provide a method of coating photographic emulsions which is an improvement over, and overcomes the noted disadvantages of the method according to Gale F. Nadeau.

Another object of the invention is to provide a method of coating allowing high coating speeds and in which the difficulties of the repellency of the coating and the rupture of the coating bead are avoided.

Still another object of the invention is to provide a method of coating which can very easily be applied to existing coating devices, and necessitates but few constructive modifications of such existing devices.

Yet another object of the invention is to provide a method of electrical coating which is suited for the coating of liquid compositions on electrically non-insulating supports. Under electrically non-insulating supports are to be understood, in the present case, supports which have a surface resistivity which is less than 10 ohm/ square cm. Such supports may be actually non-insulating supports having the said surface resistivity, as well as insulating supports which are provided at least on one side with at least one conductive layer having the mentioned surface resistance.

It may appear peculiar that the conductive properties of an actually non-insulating support are indicated by its surface resistivity. In the present case, however, it is the surface condition of the support which is important, as will be explained further in the description, and therefore the resistivity is indicated as the surface resistivity.

An object of the present invention is also the provision of a method which is particularly suited for multiple coating, i.e. the coating of more layers, one after another, in which a layer is coated before the foregoing has been dried.

According to the present invention, the method of coating a liquid composition on the surface of a support, comprises flowing said liquid composition from a supply of said liquid composition in a continuous liquid mass onto said surface, and establishing an electrical potential difference between the supply of said liquid composition and an electrically conductive member extending the width of the coating surface and closely spaced relative to or in contact with the back side of said support to increase the contact area and the adhesion between the liquid composition to be coated and the surface of the support. Under the back side should be understood, the side of the support opposite to the side being coated.

According to a particular embodiment, the method of the invention comprises the use of an electrically conductive member, the length of which is slightly smaller than the width of the support, to reduce coating on the margins of the support.

The invention will now be further described in terms of non-limitative illustrative embodiments of the accompanying drawings, of which:

FIG. 1 is a diagrammatic vertical sectional view of an apparatus which embodies the present invention, taken on the line 11 of FIG. 2.

FIG. 2 is a diagrammatic horizontal sectional view of the apparatus, taken on the line 22 of FIG. 1, the air-knife being omitted.

FIG. 3 is a sectional view representing the electrostatic field.

FIG. 4 is a diagrammatic view of a coating apparatus, the coating roller of which is provided with an insulating layer.

FIG. 5 is a diagrammatic view of a kiss coating apparatus.

FIG. 1 and FIG. 2 represent two diagrammatic sectional views of a bead or meniscus coater which is frequently used for the application of silver halide solutions in the photographic industry.

An electrically insulating support, in this case a triacetate support 11, is passed over idler roller 12 to the backing roller 13. This backing roller is a free-turning metal roller which is mounted in an insulated way by means of the bearings 14-15, and which is so arranged as to let exist a meniscus between the surface of the support 11 and the light-sensitive photographic emulsion 16 in the coating pan 17. Means are provided (not shown in the figures) for keeping the level of the emulsion 16 rigourously constant. Metering of coating is accomplished by an air-knife 18 which provides a non-turbulent air blast along which the web plus coating is passed. Variations in air volume, air pressure and angle of incidence are used to control the coating thickness. The film support is further passed with its back side over idler rollers 19 and 20 to a drying section.

The metal coating pan 17, and thereby the coating emulsion contained therein, grounded, whilst the insulated backing roller is connected to one output terminal of the potential source 21. The other terminal of said source is connected to the ground.

The potential difference which is thus applied between the backing roller and the supply of the emulsion to be coated, creates an electrostatic field, represented by the broken lines 22, which traverses the support 11 and which increases the contact area between the surface of the support and the coating emulsion 16 as indicated by the dotted lines 23 and 24, and also increases the adhesion between the emulsion and the said surface (FIG. 3).

The electrostatic field traversing the support has also a very favourable action on the neutralization of the occasional charge distributions in and on the surface of the support being coated, and in practice any previous treatment comprising the use of flows of ionized air or the pressing between grounded roller pairs has revealed itself as superfluous.

Further it appears that many of the commonly used spreading agents and surfactants in the emulsion may be omitted, without disadvantageously influencing the flow characteristics of the liquid emulsion, and without reducing the adhesion of the emulsion to the support.

In some cases, when not the highest possible coating speeds are aimed at, it has been possible to omit the addition of any of the said spreading agents to the liquid emulsion.

The proceeding of the coating has been examined by means of high speed photography. It was observed that the coating head is of extreme stability and, moreover, that the coating head is considerably thicker than if the coating was performed without applying a potential difference. Although the coating bead is thicker, the final coat weight remains unchanged, since more emulsion is removed by the air-knife.

The potential source 2.1 may be any electric or electronic power supply, capable of delivering voltages with frequencies ranging from DC. to 10' cycles, and with values situated between 50 and 1000 volts. Said values are not limitative and e.g. higher voltages may be applied, provided however, that arcing or ionization does not occur between the backing roller and the emulsion supply, or at any other place of the apparatus.

The voltage of the source 21 may undergo slight variations without influencing the coating process. At any rate, the said variations shall not exceed a given value, since otherwise the thickness of the coated layers may undergo variations.

The amount of current which may be drawn from said sources is at most very small, since no complete or closed electrical circuit is created. Leakage currents which exist at the insulating areas and currents which arise owing to disturbances in the electrostatic field caused by static charges in the supports, on the surface thereof, and

and the air-knife areby changes in the dielectric properties of the supports, result in a total current which in practice does not exceed 10 mA.

The potential source 21 may consist of a simple separation transformer connected to the electrical supply main, followed by a metal rectifier, a smoothing filter and a series resistance, or of an electronically stabilized power supply capable of delivering voltages which can continuously or step-wise be changed within a considerable range, of a LP. or HP. generator.

It has to be noticed that in case a DC. potential difference is applied, the conductive member at the back side of the support may be positive as well as negative in respect of the liquid supply to be coated. In some cases, however, a given polarity yields slightly better results than the opposite polarity, and the right choice in such cases has to be made empirically by practical tests.

It appears from FIGURE 2, that the length of the backing roller 13 is somewhat smaller than the width of the support 11, so as to let the support extend with its margins aside said roller.

The width of the extending margin portions of the support amounts to a few millimetres. The said extending portions prevent the liquid emulsion from touching the side walls of the backing roller and consequently establishing a conductive path between the roller and the emulsion. Another consequence of said measure is that almost no electrostatic field traverses the said margin portions and consequently little if any emulsion is coated on the margins of the support. Thereby the loss of emulsion in coating edges, which are normally removed in a subsequent coating operation, is avoided. The coated part of the surface of the support is indicated in FIG. 2 by the part 26 hatched in broken lines.

The described embodiment related to the coating of liquid compositions on electrically non-conductive supports such as paper, polystyrene, polyethylene terephthalate, and any other web material with a surface resistivity greater than 10 ohm/inch.

In case e.g. a very thin paper support is coated with a substance of very low viscosity, it may happen that the said substance penetrates locally through the support and makes the support non-insulating as defined on page 4 of the description. This conductivity causes a disturbance of the electrostatic field which no longer will be concentrated between the emulsion supply and the backing roller but will disperse within the support, and moreover, said conductivity gives rise to an electrically conductive path between the backing roller and the emulsion supply. The said conductive path causes a flow of electric current, which unfavourably influences the photographic properties of the resultant photographic film. This conductive path also causes a decrease in the potential difference, resulting in changes in the meniscus.

Dissipation of the electrostatic field will also result if electrically insulating supports are provided with electrically non-insulating layers. Such supports are e.g. negative and positive multicolor photographic films comprising a support which on its back side is provided with an antihalation layer containing graphite for rendering such layer conductive so as to avoid electrostatic charge distributions, supports provided with a conductive antistress layer on their back side, supports which are provided on the surface to be coated with a previous layer which has not yet completely dried and therefore is still conductive, etc.

In the case a conductive layer is provided on the back side of the support, an electric current will flow through the said conductive layer to the ground at those places where the coated support is conveyed with its back side over the metal idler rollers.

According to the invention, any dissipation of the electrostatic field between the backing roller and the emulsion supply in the coating of an electrically conductive support, as well as any current flow, is avoided by prO- viding the metal coating roller with an insulating layer and by applying an A.C. potential difference between the emulsion supply and the coating roller.

The insulating layer may consist of any suitable material, e.g. polyamides such as Akulon (trademark for a polyamide, marketed by AKU N.V., Arnhem, Holland), polymers such as P.V.C., Teflon (trademark for polytetrafluoroethylene, marketed by E. I. du Pont de Nemours and Co., Wilmington, Del.), Plexiglas (trademark for polymethyl methacrylate, marketed by Rohm and Haas, Philadelphia, Pa.) etc. The thickness of the said layer is in the order of magnitude of some tenths of a millimetre.

The use of an A.C. is imperative in the case Where the support is conductive, since the electrical equivalent of the arrangement just described is the parallel connection of a resistor with a capacitor, which is connected to the voltage source (the capacitor representing the capacitance between the backing roller and the emulsion supply and the resistor representing the resistance of the electrically conducting support in longitudinal direction).

The better the conductivity of the support, the higher the frequency of the source will have to be for concentrating most of the electrical energy in the capacitive member of the equivalent parallel connection, and for consequently yielding a high efiicient process.

An apparatus for processing conductive webs or supports in accordance with the invention is represented in FIG. 4. The insulated metal backing roller 13' is provided with a ground and polished Teflon layer 27. The height of the roller relative to the surface of the emulsion 16 is again so adjusted as to permit the existence of a meniscus between the support 11 and the emulsion. The roller 13 is so mounted as to be electrically insulated and is connected to the electrical potential source 21. The thickness of the coated layer is in the present embodiment controlled by the stainless steel roller 28 mounted at a fixed distance from the periphery of the backing roller 13.

The A.C. voltage source 21 is connected between the backing roller 13 and the pan 17 containing the emulsion supply 16. The pan as well as the metal roller 28 are grounded.

It has been stated in practice, that for the coating of a layer onto a relative good conductive support, e.g. the coating of an anti-stress layer on a light-sensitive panchromatic emulsion layer which has been coated immediately before, or the coating of a photographic emulsion on the electrically conductive graphite substrate layer of a negative black-and-white cine film, frequencies up from 10,000 cycles give very good results.

A third embodiment of the invention representing a kiss coating device, is shown in FIG. 5. Here, the arrangement comprises the coating roller 13", the pan 17 containing the liquid emulsion 16, and the idler roller 29. The thickness of the coated layer is controlled by the doctor knife 31. According to the invention a conductive member 30, consisting of a copper bar is provided at a distance of a few millimetres above the coating roller 13. The said bar extends over the full width of the suppport 11, and its length equals the length of the coating roller 13. The potential source 21 is connected between the said bar and the pan 17.

As in the previous embodiments, an electrostatic field is created here also, traversing the support 11 from the bar 30 towards the roller 13, increasing thereby the contact area of the emulsion between the roller 13 and the surface of the support, and enhancing the adhesion.

Since there is no direct contact between the bar 30 and the support 11, no conductive path can arise between the coating roller 13 and the said bar, and therefore the present embodiment is also suited for the coating of conductive supports as defined hereinbefore.

It is clear that the described devices differ 'but very little from the commonly used coating devices, and that simple measures suffice for adapting existing coating devices for the practice of the coating method according to the invention.

These measures include an insulated mounting of the bearings of the coating roller, the provision of a contact strip for contacting the said insulated roller, the covering of the roller surface with an insulating layer, the Provision of a conductive bar closely adjacent the periphery of the coating roller, etc.

The coating pan as well as all other parts of the apparatus are grounded, so that protective screening of the coating roller or the conductive element adjacent to the back side of the support can be realized in a most simple way.

The examples given hereinbefore relate to the coating of photographic emulsions. It is clear that the invention is not limited to a method of coating merely light-sensitive photographic emulsions, but includes the coating of all other liquid compositions having a specific conductivity of the magnitude of at least 10- mhos per cc.

The invention can also be practised for the coating of anti-stress layers, anti-halation layers, color filter layers etc. in the production of photographic material, for the coating of zinc oxide and similar photoconductive layers in the production of electrophotographic recording material, for the coating of thermo-sensitive layers in the production of thermophotographic recording material, for the coating of image-receiving layers in the production of an image-receiving material suited for being used in the silver halide diffusion transfer processes, for the coating of electrically conductive layers on supports with very good insulating properties in the production of electrostatic recording material, for the coating of stripping layers in the production of photographic stripping films for use in photomechanic reproduction processes, for the coacting of magnetizable layers in the production of magnetic recording tape, for the coating of subbing layers, for the production of X-ray film, for the coating of plastics and resins on flexible supporting webs, which themselves may or may not be plastics, etc.

Finally it should be understood, that the coating method of the invention is not limited to the coating techniques of the described example, but is also applicable to other coating techniques including spread coating methods wherein use is made of a rubber spreader or a brush spreader, roller coating methods using a calender coater, a reverse roller coater, a doctor knife or a trailing blade, extrusion coaters, etc.

What we claim is:

1. The method of coating a flowa-ble liquid composition having a specific conductivity of at least 10- mhos/cc. onto the surface of a moving web of insulating material which comprises:

(1) Passing said web along a path such that the back surface thereof moves in at least close proximity to an electrically conductive member extending substantially the width of the web;

(2) Flowing said liquid composition in a continuous liquid mass onto said surface from a supply thereof maintained on the opposite surface of said web from said member in general alignment therewith; and

(3) Establishing and maintaining an electrical field between said liquid composition and said member of a potential insuflicient to cause ionization therebetween but suflicient to increase the contact area and adhesion between said liquid composition and the web surface.

2. The method of coating according to claim 1, wherein the eifective length of said electrical conductor is slightly less than the width of the web support, to reduce coating on the margins of the support.

3. The method of coating according to claim 2, wherein the electrical conductor member is a conductive coating roller which conveys the web support to the position at which the liquid composition is coated thereon.

4. The method of coating according to claim 1, wherein the said potential source delivers a direct current potential.

5. The method of coating a flowable liquid composition having a specific conductivity of at least 10 mhos/cc. onto the surface of a moving web of electrically conductive material, which comprises:

(1) Passing said web along a path such that the back surface thereof moves in at least close proximity to an electrically conductive member extending substantially the width of the web while maintaining said material insulated from said member;

(2) Flowing said liquid composition in a continuous liquid mass onto said surface from a supply thereof maintained on the opposite surface of said web from said member in general alignment therewith; and

(3) Establishing and maintaining an electrical field between said liquid composition and said member of a potential insufiicient to cause ionization therebetween but sufiicient to increase the contact area and adhesion between said liquid composition and the web surface, said potenial being an AC. potential having a frequency such that the time per cycle is less than the charge decay time of said conductive web.

6. The method of coating according to claim 5, wherein the said liquid composition is a liquid photographic emulson.

8 7. The method of claim 4' wherein said frequency is greater than 1,000 cycles.

8. The method of claim 5 wherein the maximum current loss in said field is not greater than 10 ma.

References Cited UNITED STATES PATENTS 2,520,504 8/1950 Hooper 101-426 2,558,900 7/ 1951 Hooper 101-219 2,898,279 7/1959 Metcalfe et al. 204-181 2,952,559 9/1960 Nadeau 117-34 3,063,868 11/1962, Brandsma et al 117-102 3,102,045 8/ 1963 Metcalfe et a1 117-37 3,203,395 8/1965 Liller 118-637 3,005,726 10/1961 Olson 117-37 3,053,179 9/1962 Reithel 117-934 X 2,052,131 8/1936 Chappell 117-93.4 X

MURRAY KATZ, Primary Examiner E. J. CABIC, Assistant Examiner US. Cl. X.R. 

1. THE METHOD OF COATING A FLOWABLE LIQUID COMPOSITION HAVING A SPECIFIC CONDUCTIVITY OF AT LEAST 10**-10 MHOS/CC. ONTO THE SURFACE OF A MOVING WEB OF INSULATING MATERIAL WHICH COMPRISES: (1) PASSING SAID WEB ALONG A PATH SUCH THAT THE BACK SURFACE THEREOF MOVES IN AT LEAST CLOSE PROXIMITY TO AN ELECTRICALLY CONDUCTIVE MEMBER EXTENDING SUBSTANTIALLY THE WIDTH OF THE WEB; (2) FLOWING SAID LIQUID COMPOSITION IN A CONTINUOUS LIQUID MASS ONTO SAID SURFACE FROM A SUPPLY THEREOF MAINTAINED ON THE OPPOSITE SURFACE OF SAID WEB FROM SAID MEMBER IN GENERAL ALIGHMENT THEREWITH; AND (3) ESTABLISHING AND MAINTAINING AN ELECTRICAL FIELD BETWEEN SAID LIQUID COMPOSITION AND SAID MEMBER OF A POTENTIAL INSUFFICIENT TO CAUSE IONIZATION THEREBETWEEN BUT SUFFICIENT TO INCREASE THE CONTACT AREA AND ADHESION BETWEEN SAID LIQUID COMPOSITION AND THE WEB SURFACE. 